Organic isocyanate recovery process by distillation with a tar-thinning agent and a stripping agent

ABSTRACT

Organic isocyanates can be recovered with beneficial ease and efficiency from crude isocyanate compositions containing tar (residue component) and other contaminants (e.g., compositions as obtained by phosgenation of an amine in a solvent) by a process which includes adding a suitable amount of a useful tar-thinning agent and distilling the resulting mixture either batchwise or continuously in the presence of a stripping agent so that the distillate contains most of the organic isocyanate and the still heel contains the tar and tar-thinning agent.

United States @etent 1191 Erwin et all.

[ 1 Apr. 24, 1973 ORGANEC ISOCYANATE RECOVERY PROCESS BY DHSTHLLATHONWITH A TAR-THINNING AGENT AND A STRIPPING AGENT [75] Inventors: Carl F.Irwin, New Castle, Del.; William T. Muncaster, Woodstown, NJ.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: jam. 4, 1971 [21] Appl. No.: 103,734

Related US. Application Data [63] Continuation-impart of Ser. No.39,779, May 22,

1970, abandoned.

[52] US. Cl. ..203/8, 203/49, 203/52, 203/57, 203/60, 203/62, 203/63,203/67,

[51] Int. Cl. ..C07c 119/04, BOld 3/34 [58] Field of Search ..203/49,52, 57, 62, 203/63, 60, 67, 68, 69, 71, 73, 8, 70;

260/453 SP, 453 PH [56] References Cited UNITED STATES PATENTS 2,884,360Bloom et a1. ..260/453 SP 2,680,128 1/1954 Slocombe et a1. ..260/453 PH3,169,141 2/1965 Kober et a1 .203/63 3,658,656 4/1972 Adica et a1.....203/70 3,436,318 4/1969 Glass ....203/52 3,259,555 7/1966 Lankton eta1. ....203/6O 2,884,359 4/1959 Bloom et a1. ....203/52 2,884,361 4/1959Bloom et a1. ....203/60 2,884,362 4/1959 Bloom et a1. ....203/802,884,363 4/1959 Bloom et a1. ..203/57 2,885,420 5/1959 Spiegler..260/453 SP Primary Examiner-Wilbur L. Bascomb, Jr. Attorney MelvilleJ. Hayes [57] ABSTRACT Organic isocyanates can be recovered withbeneficial ease and efficiency from crude isocyanate compositionscontaining tar (residue component) and other contaminants (e.g.,compositions as obtained by phosgenation of an amine in a solvent) by aprocess which includes adding a suitable amount of a useful tar-thinningagent and distilling the resulting mixture either batchwise orcontinuously in the presence of a stripping agent so that the distillatecontains most of the organic isocyanate and the still heel contains thetar and tar-thinning agent.

8 Claims, 2 Drawing Figures Patented April 24,, 1973 3,729,386

FEE-l l8 si s4 s2 21* 2? s 2 9 53; L

INVENTORS CARL FRANCIS IRWIN WILLIAM THOMAS MUNCASTER ORGANIC ISOCYANATERECOVERY PROCESS BY DESTILLATION WTTH A TAR-THINNING AGENT AND ASTRIPPIING AGENT CROSS-REFERENCE TO RELATED APPLICATION This is acontinuation-in-part of our patent application Ser. No. 39,779 filed May22, 1970 (now abandoned).

BACKGROUND OF THE INVENTION This invention relates to a process forrecovering organic isocyanates from crude organic isocyanatecompositions including those obtained by phosgenation of an amine in asolvent.

Organic isocyanates such as toluene diisocyanate (TDI) are known to beuseful in the manufacture of flexible and rigid polyurethane foams. Theycan be produced by the phosgenation of a primary amine corresponding tothe desired isocyanate at an elevated temperature in an inert highboiling solvent. The phosgenation reaction yields a crude organicisocyante solution including solvent, tar, and minor amounts of phosgeneand hydrochloric acid. The word tar is often used by workers in the artto mean the substantially nonvolatile residue component which forms thestill heel after distilling off the volatile impurities and theisocyanate.

A commercial process for recovering isocyanates from the crude organicisocyanate solutions involves the sequential distillation of the crudesolution to remove the minor amounts of phosgene and hydrochloric acid,the solvent, and then the organic isocyanate, leaving as a distillationheel a viscous, tarry material still containing a substantial amount ofisocyanate. This distillation heel can contain as much as 30 to 50percent by weight of organic isocyanate.

It has usually been considered economically impractical to reduce theconcentration of isocyanate in this tarry heel by ordinary distillationmethods. Attempted distillation of isocyanate from the tarry heel inconventional equipment is a slow process; column bottoms are often heldup for periods of about 8 hours at high temperatures. During suchperiods, considerable polymerization and degradation can take place,resulting in an increase in viscosity which can cause deposition of asolid material on the walls of pipes and tanks. The problem becomesworse as the tarry heels are further processed to recover additionalisocyanate. This procedure results in losses of substantial amounts ofisocyanate formed during the phosgenation step, and requires frequentequipment shutdowns for cleaning. Mechanically assisted evaporators,such as wiped film evaporators, have been employed in an attempt toefficient recovery of residual organic isocyanate from the tarry heel.Such mechanically assisted evaporators are, however, extremelyexpensive.

U.S. Pat. No. 2,680,128 suggests the distillation of crude isocyanate inthe presence of a plasticizing agent to increase the yield. Thedistillation temperature needed to obtain a desirable yield, however, isrelatively high; thus, the distillation heel tends to polymerize andsolidify. The solidified heel is unmanageable, dif-' ficult to removefrom the process equipment, and frequently results in equipmentblockage.

There is, therefore, a need for an efficient and inexpensive method forrecovering organic isocyanate from the crude organic isocyanate solutionwhereby one can reduce the amount of isocyanate lost in the recoveryprocess, obtain a manageable residue after the recovery of isocyanate,and avoid equipment blockage.

SUMMARY OF THE INVENTION The present invention provides an organicisocyanate recovery process which comprises A. providing a crude organicisocyanate composition containing a substantially nonvolatile residuecomponent as an impurity,

B. adding to said isocyanate composition a tar-thinning agent in anamount such that the weight ratio of tar-thinning agent to said residuecomponent in the resulting mixture is about 0.2/1 to 5/1, thetarthinning agent being an organic liquid which dissolves the residuecomponent and has a boiling point at least 50C. above that of theorganic isocyanate component, and

C. subjecting the resulting mixture to distillation conditions in thepresence of a stripping agent for the organic isocyanate whereby thedistillate contains a major proportion of the organic isocyanate and thestill heel contains the residue component and the tar-thinning agent.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow sheet diagram showing(as described in detail below) the steps of preparing a crude organicisocyanate, distilling to remove phosgene and hydrochloric acid, addinga tar-thinning agent, distilling to remove the solvent, and strippingisocyanate from the distillation heel.

FIG. 2 is a flow sheet diagram of a preferred embodiment showing thesteps of preparing a crude organic isocyanate, distilling to removephosgene and hydrochloric acid and then the solvent, adding atarthinning agent, distilling off the bulk of the isocyanate, andstripping isocyanate from the remaining distillation heel.

DESCRIPTION OF PREFERRED EMBODIMENTS In certain preferred embodiments ofthe present process, the crude composition provided in Step (A) is acomposition resulting from the preparation of an organic isocyanate bythe phosgenation of an amine in an organic solvent; the compositioncontains said solvent plus a small amount of phosgene and hydrochloricacid as impurities; and the phosgene and hydrochloric acid are removedfrom the composition by distillation, for example, in the presence of astripping agent prior to Step (B).

It is known that stripping procedures sometimes comprise removal ofvolatile components from a composition by passing a gas through it, andsometimes they comprise addingto the composition a volatile liquid whichcodistills with the volatile components already in the composition.

We usually prefer to remove the solvent from the composition bydistillation prior to Step (B) as shown in FIG. 2. However, it can alsobe removed just after Step (B) as shown in FIG. 1.

In one very useful embodiment of the process, the organic isocyanate istoluene diisocyanate and the solvent to be removed isorthodichlorobenzene.

Among the most useful tar-thinning agents are aromatic hydrocarbonshaving less than 35 weight percent saturates, esters of aromatic acidshaving alcohol radicals of C -C aromatic ketones having the carbonylradical linked to two aryl radicals, aromatic ethers having the etheroxygen linked to two aryl radicals, and blends .of two or more suchmaterials. Especially preferred is an aromatic petroleum oil having lessthan 35 weight percent saturates. Also useful are fully aromaticsulfides such as diphenyl sulfide, and fully aromatic sulfones such asdiphenyl sulfone. If the boiling point of the tar-thinning agent is notenough greater than that of the isocyanate, an excessive amount of thethinning agent is distilled with the isocyanate. Some of the bestthinning agents have a boiling point at least lC. above that of theisocyanate at atmospheric pressure. Useful tar-thinning agents areillustrated as follows:

Representative aromatic hydrocarbons include aphenylnaphthalene and itsmixtures with B-phenylnaphthalene, low melting grades of polystyrene,isomeric mixtures of terphenyl and substituted derivatives of thesehydrocarbons containing chloro-, bromo-, alkyl, alkoxy, or nitrogroups.A particularly preferred class of aromatic hydrocarbons are highlyaromatic petroleum oils which are often used as rubber process oils orrubber extending oils. To be useful in the present process, thepetroleum oils should contain less than 35 percent, and preferably lessthan percent, by weight of saturates as determined by the Clay-Gelmethod of molecular analysis (ASTM D-2007). This analysis is commonlyused to characterize petroleum oils and such information is usuallyavailable from suppliers of such oils.

Representative esters of aromatic acids where the alcohol radicalscontain one to 12 carbon atoms include diisooctyl phthalate, diethylphthalate, trioctyl trimellitate, diisobutyl terephthalate, diethyleneglycol dibenzoate and phenyl benzoate. Esters of aromatic acidssubstituted with chloro, bromo, alkyl, alkoxy, or nitro groups can alsobe used. Phthalate esters are preferred because they are readilyavailable and economical to use.

The aromatic ketones should be fully aromatic in that the carbonylradical of the ketone is linked to two aryl radicals. Representativeketones include benzophenone, aand B-naphthyl phenyl ketones andderivatives substituted as described for the esters.

The aromatic ethers should be fully aromatic in that the ether oxygen islinked to two aryl radicals. Representative compounds include aandB-naphthyl phenyl ethers, diphenoxybenzene isomer mixtures, lowerpolyphenyl ethers and derivatives substituted as described for theesters.

When using a blend of two or more thinning agents, the initial boilingpoint of the blend should be at least 50C. above that of the isocyanate.A thinning agent is used which does not undergo any hannfuldecomposition or harmful reactions under the process conditions with theother materials present.

In many cases we prefer that the weight ratio of tarthinning agent toresidue component is about 0.7/1 to 1.5/1. The best ratio to use in aparticular application depends on such factors as the nature of thethinning agent and the isocyanate components, the extent of isocyanaterecovery required, and the still heel viscosity desired.

Step (C) is preferably carried out at a temperature of about l00-l C.and a pressure of about 05-50 mm. Hg. In some embodiments, the strippingagent used in removing the phosgene and hydrochloric acid is nitrogen,the stripping agent used in removing the organic isocyanate in Step (C)is orthodichlorobenzene, and Step (C) is carried out at a temperature ofabout l30l65C. and a pressure of about 2-20 mm. Hg.

The stripping agents used in the isocyanate stripping still are usuallylower boiling that the isocyanate being recovered, and preferably aresufficiently lower boiling to be separated easily from the isocyanatebeing recovered. In addition, these agents (like the thinning agents)should not undergo any harmful reactions under the process conditions.Representative materials are gases such as nitrogen and carbon dioxide;low boiling solvents such as benzene, hexane, carbon tetrachloride; andhigher boiling solvents such as toluene, chlorobenzene, ando-dichlorobenzene. Solvents suitable for phosgenation are particularlyeconomical to use in commercial operation of this process as only onesolvent is required for phosgenation and stripping. In addition, suchsolvents are usually readily condensed along with recovered isocyanateduring codistillation.

Step (C) can be carried out as a batch procedure; or it can be performedas a continuous procedure. For example, Step (C) can comprise passing acontinuous stream of the mixture obtained in Step (B) and the strippingagent into distillation means capable of continuously removing a majorproportion of the organic isocyanate, and passing a continuous stream ofthe distilled isocyanate and stripping agent out of the distillationmeans. One skilled in the art, after reading the present disclosure,will be able to select for a particular application the distillationmeans and conditions suitable for use in the continuous procedure aswell as in the batch procedure. Thus, one can carry out the Step (C)stripping operation continuously by employing a plurality ofcounter-current and/or co-current stages. The counter-current stages canemploy such counter-current strippers as distillation columns,falling-film evaporators, and vacuum stripping vessels operatedcounter-currently. The co-current stages can employ such co-currentstrippers as two or more vacuum stripping vessels operated co-currently,or turbannular strippers. The latter can be used in combination withjacketed kettles; the vapor and liquid from the stripper enter near thetop of the kettle and the vapors flow to a condenser for recovery. Theliquid in the bottom of the kettle can be further reduced in isocyanatecontent be feeding additional stripping agent to the bottom of thekettle. Continuous stripping is especially preferred when it isimportant to use the least possible amount of stripping agent based onthe amount of isocyanate recovered.

Referring to the diagram shown in FIG. 1 toluene diamine, phosgene, andan organic solvent such as orthodichlorobenzene (ODCB), are fed throughconduits l, 2, and 3, respectively, into a conventional phosgenationreactor 4. The phosgenation reaction product consists mainly of TDI(toluene diisocyanate), ODCB, minor amounts of phosgene and hydrochloricacid, and tar. (as defined above). The phosgenation reaction mixture isfed through conduit 5 to a conventional stripping still 6 where theminor amounts of phosgene and hydrochloric acid are stripped away byforcing a gas such as nitrogen into stripping still 6 through conduit 7,and removing from the top of the stripping still through conduit 8 thephosgene and hydrochloric acid along with the stripping gas. After thephosgene and hydrochloric acid are removed, the remaining solution isfed through conduit 9 into a conventional solvent distillation still 10.A tar-thinning agent, namely, a highly aromatic petroleum oil havingless than 15 weight percent saturates, is added to the solution throughconduit 1 1, which joins conduit 9, and becomes a part of the solutionfed intothe solvent distillation still 10. In the solvent distillationstill, solvent is removed through conduit 12, leaving a solution of TDI,tar, and the tar-thinning agent. This solution is fed through conduit 13into stripping still 14; a stripping agent such as ODCB is fed into thestill through conduit 15, is forced through the solution, and is removedfrom the stripping still through conduit 16 along with TDI. Thetemperature in the still is maintained at about 130 to 160C. under apressure of about 2 to mm. of mercury. The ODCB and TDI vapors are thensent to a condenser and subsequently separated by distillation accordingto conventional techniques. About 98 percent of the TDI produced isrecovered. The ODCB recovered can be reused by recirculating it to thephosgenation reactor 4. The material remaining in the bottom of thestill contains about 9% TDI, 45.5 percent tar and about 45.5 percenttar-thinning agent. This material has a viscosity of less than 1,000centipoises at 130 C. when formed, and a viscosity of about 2,500centipoises two hours after withdrawal from the stripping still 14through conduit 17; it can be further processed to recover metatoluenediamine.

In the flow sheet diagram of FIG. 2, representing a preferredembodiment, toluene diamine, phosgene and an organic solvent such asODCB are fed through conduits 18, 19 and 20 into a conventionalphosgenation reactor 21. The phosgenation reaction product is fedthrough conduit 22 into a conventional stripping still 23 where phosgeneand hydrochloric acid are removed by stripping; this is accomplished byforcing a gas such as nitrogen through conduit 24 and through thesolution, and removing from the top of the still through conduit 25phosgene, hydrochloric acid and nitrogen gas. The remaining solution isfed through conduit 26 into a conventional solvent distillation still 27where ODCB is removed from the top of the still through conduit 28. Theremaining solution is composed of about 90% TDI and l0percent residuecomponent. The solution is fed through conduit 29 into a conventionalTDI distillation still 30. A tar-thinning agent, (e.g., a highlyaromatic petroleum oil having less than weight percent saturates) isadded through conduit 31, which feeds it into line 29 and mixes it withthe solution before it enters the TDI distillation still 30. Indistillation still 30, about 90 percent of the TDI produced is removedfrom the top through conduit 32, leaving a still heel composed of about33% TDI, 33 percent tar, and 34 percent tar-thinning agent. The stillheel is fed through conduit 33 into stripping still 34 where a strippingagent such as ODCB is fed through the heel and is removed from the topthrough conduit 36 along with TDI. The solution remaining in the bottomof still 34 is composed of about 9% TDI, 45.5 percent tar, and 45.5percent tar-thinning agent; it is pumped from the stripping stillthrough conduit 37 and is further processed to recover metatoluenediamine. The total amount of isocyanate recovered is about 98 percent ofthat produced.

The process shown in in FIG. 2 can be modified if desired by passing thestill heel from conduit 33 through a short residence time heat exchangerwhich is maintained under superatmospheric pressure, and heating thestill heel just before it enters still 34 so that most of the TDI in theheel will vaporize and distill about the same instant the heel entersthe still.

The use of a tar-thinning agent in accordance with this invention addsversatility to the process of recovering isocyanates as evidenced by theabove embodiments. In the first embodiment as shown in FIG. 1, thetar-thinning agent maintains the crude organic isocyanate composition asa solution, and the process proceeds without difficulty by the steps offractionally distilling the minor amounts of phosgene and hydrochloricacid, and then the solvent; and then removing about 98 percent of theisocyanate in a stripping still, leaving a pumpable tar that is easy tomanage.

In the preferred embodiment shown in FIG. 2, the tar-thinning agentmaintains the crude organic isocyanate composition as a solution, andthe process proceeds without difficulty by the steps of removing theminor amounts of phosgene and hydrochloric acid, and then the solvent;and then much of the isocyanate is removed by practical distillationtechniques which permit the recovery at this stage of about percent ofisocyanate prepared. Polymerization of the distillation heel isminimized due to the presence of the tar-thinning agent; this permitsthe distillation heel to be further processed by feeding it to astripping still where additional isocyanate is recovered, giving anoverall yield of about 98 percent. Furthermore, the still heel has a lowenough viscosity so that it is easy to remove and to process further.

The use of a tar-thinning agent in combination with the isocyanatestripping still provides an improved process for recovering isocyanates,resulting in increased yields of isocyanates, and leaving a still heelwhich has a relatively low viscosity and is easy to remove from thestill. By stripping isocyanate from the crude reaction mass instead ofrecovering it by fractional distillation, lower temperatures can beused; this reduces the amount of polymerization that takes place in thestill heel, and results in a lower heel viscosity. The lower theviscosity of the tar the greater the amount of entrapped organicisocyanate recovered. There is a point, however, where the addition ofmore tar-thinning agent is more costly than the gain received fromrecovering a little more isocyanate. In the FIG. 2 embodiment, theaddition of the tar-thinning agent just prior to the removal of the bulkof the isocyanate in the TDI still reduces the residence time in thestill and dilutes the heel, making it very manageable for furtherprocessing in the stripping still.

When the high-boiling tar-thinning agent does not undergo hydrolysis andis inert toward aromatic amines as is the case with the preferredaromatic petroleum oils, the still heels from this process can bereadily hydrolyzed to recover diamines which can be returned to thephosgenation step. For example, one can employ hydrolysis with water ina pressure vessel as disclosed in German Pat. No. 1,013,281; orhydrolysis with superheated steam as disclosed in US. Pat. No.3,225,094. Following hydrolysis, the diamines can be separated from thetar-thinning agent by distillation or other conventional separationprocedures.

Among the isocyanates which can be recovered by the process of thisinvention are: aromatic isocyanates containing 7-12 carbon atoms;aliphatic isocyanates containing 7-14 carbon atoms; and cycloaliphaticisocyanates containing 7-15 carbon atoms. Representative examples ofaromatic isocyanates containing 7-12 carbon atoms include: 1,3- and1,4-phenylene diisocyanates, 2,4 and 2,6-tolylene diisocyanates, 3,4-dichlorophenyl isocyanate, o, mand p-tolyl isocyanates, aand B-naphthylisocyanates, 1,4 and 1,5naphthylene diisocyanates, durene diisocyanate,and 4-isopropylphenyl isocyanate.

Representative aliphatic isocyanates containing 7-14 carbon atomsinclude: 1,6-hexamethylene diisocyanate, a,a-xylylene diisocyanate,1,12- dodecamethylene diisocyanate, dodecyl isocyanate, lysinediisocyanate, and B,B'-diisocyanatodiethyl carbonate.

Representative cycloaliphatic isocyantes containing 7-15 carbon atomsinclude: cyclohexyl isocyanate, 1,3- and 1,4-cyclohexylenediisocyanates, l-methyl-2,4 and 2,6-cyclohexylene diisocyanates, 4,4-methylenebis (cyclohexyl isocyanate), and 4-phenyl-' cyclohexylisocyanate.

This invention is illustrated further by the following examples. Allamounts are by weight unless otherwise indicated.

EXAMPLE 1 A crude solution of TDI (toluene diisocyanate) is prepared bythe phosgenation of m-tolylene diamine in ODCB (orthodichlorobenzene).The crude solution is subjected to a fractional distillation process toremove minor amounts of phosgene and hydrochloric acid, and then ODCB.The heel is a liquid containing about 88 percent distillablediisocyanate (80 percent 2,4-isomer and 20 percent 2,6-isomer) and 12percent nonvolatile residue.

The heel is first mixed with ODCB as a stripping agent. There are 167parts of heel and 620 parts of ODCB present. This mixture is added to anagitated, vacuum stripping still containing 20 parts of a petroleum oil;there is one part of tar-thinning agent per part of residue. Theaddition is uniformly made over a period of about 3 hours. The petroleumoil has an initial boiling point of 393C. (boiling point of TDI being250C.) and a molecular analysis by the Clay-Gel method (ASTM D-2007) of23 percent polar compounds, 70.3 percent aromatics, and 6.7 percentsaturates.

The stripping still is operated about 160C. under a pressure of mm. Hg.,and 143.4 parts of TDI are stripped and recovered. This corresponds to arecovery of 97.5 percent of the volatile diisocyanate available in thestarting crude solution. The heel remaining after the stripping processis fluid at 140C. and is easily removed from the stripping still bypouring.

EXAMPLE 2 A crude solution of TDI is prepared by the phosgenation ofrn-tolylene diamine in ODCB. The crude solution is subjected to afractional distillation process to remove minor amounts of phosgene andhydrochloric acid, and then the ODCB. The heel is a solution containing86.4% TDI percent 2,4-isomer and 20percent 2,6-isomer) and 13.6 percentnonvolatile residue.

The heel is added to an agitated vacuum stripping still containing 150parts of an aromatic petroleum oil having an initial boiling point of354C. and a molecular analysis by the Clay-Gel method (ASTM D-2007) of8.2 percent polar compounds, 78.2 aromatics and 13.6 percent saturates.The amount of petroleum oil used corresponds to 1.1 parts of oil perpart of residue. The liquid in the still is maintained at 160C. under apressure of 10 mm. Hg. while 1,000 parts of the heel are uniformly addedover a period of minutes under the surface of the oil in the still.

Distillation yields 729 parts of TDI. The temperature of the liquid inthe still is lowered to 130C. and the pressure reduced to 2 mm. Hg,resulting in the distillation of 54 additional parts of TDI.

While maintaining the temperature of the liquid at 130C. under 2 mm. Hg,140 parts of ODCB stripping agent are added to the stripping still(under the liquid in the still) at a uniform rate over a period of 30minutes. The ODCB and TDI are codistilled, yielding an additional 56parts of TD].

The total amount of TDI recovered is 839 parts out of 864 partsavailable representing a yield of 97 percent.

The still heel has a viscosity of 250 centipoises at 130C. immediatelyafter the stripping process is stopped; the viscosity increases to 1,500centipoises at 130C. after 2 hours.

EXAMPLE 3 Crude TDI prepared substantially as described in Example 2containing 88% TDI and 12 percent residue is distilled to remove minoramounts of phosgene and hydrochloric acid, and then the solvent. Thecrude solution is then distilled at 160C. and 10 mm. Hg. until the heelcontains 47.8% TDI and 52.2 percent nonvolatile residue. Then 100 partsof the heel and 45 parts of the petroleum oil described in Example 2 areplaced in an agitated vacuum stripping still; there is 0.86 part oftar-thinning agent per part of residue. The pressure is reduced to 2 mm.Hg. and the temperature is adjusted to 130C., resulting in thedistillation of 11.6 parts of TDI. While maintaining the temperature at130C. and the pressure at 2 mm. Hg., 14.4 parts of ODCB are added overabout 30 minutes at a uniform rate. An additional 17.2 parts of TDI arecodistilled with ODCB. lmmediately after the stripping, the still heelis an easily managed liquid having a viscosity of cps. at C. After 2more hours at 130C., the viscosity increases to 750 cps. at 130C. Thetotal amount of TDI recovered represents a yield of about 95.2 percent.

EXAMPLE 4 Crude TDI is processed as described in Example 2 to removeminor amounts of phosgene and hydrochloric acid and then the solvent.The remaining crude solution is further distilled at 160C. and about 10mm. Hg. until the heel contains 37.2% TDI and 62.8 percent nonvolatileresidue. Then 100 parts of heel and 13 parts of diisooctyl phthalate areplaced in an agitated vacuum stripping still (0.21 parts of tar-thinningagent per part of residue), and the temperature is adjusted to 130C. ata pressure of 2 mm. Hg, resulting in the distillation of 5.5 parts ofTDI. While maintaining the still at 130C. and 2 mm. Hg, 8.9 parts ofstripping agent (ODCB) are added at a uniform rate over about 15minutes, resulting in the codistillation of an additional 9.5 parts ofTDI. The resulting still heel is an easily managed liquid having aviscosity of 170 cps. at 140C. immediately after the stripping iscompleted. After 2 hours at 140C., the viscosity increases to about3,000 cps. at 140C. The total amount of TDI recovered represents a yieldof about 95.2 percent.

We claim:

1. An organic isocyanate recovery process which comprises A. providing acrude organic isocyanate composition containing a substantiallynonvolatile residue component as an impurity, said crude compositionbeing one (1) which has resulted from the preparation of an organicisocyanate by the phosgenation of an amine in an organic solvent, (2)which contains said solvent and had contained a small amount of phosgeneand hydrochloic acid as impurities, and (3) which has had the phosgeneand hydrochloric acid removed by distillation,

B. mixing with said isocyanate composition a tarthinnin agent in anamount such that the weight ratio of tar-thinning agent to said residuecomponent in the resulting mixture is about 0.2/1 to 5/1, thetar-thinning agent being an organic liquid which dissolves the residuecomponent and has a boiling point at least 50C. above that of theorganic isocyanate component, and the tar-thinning agent also being atleast one liquid selected from the group: aromatic hydrocarbons havingless than 35 weight percent saturates, esters of aromatic acids havingalcohol radicals of C C aromatic ketones having the carbonyl radicallinked to two aryl radicals, and aromatic ethers having the ether oxygenlinked to two aryl radicals, and

C. subjecting the resulting mixture to distillation conditions in thepresence of a stripping agent for the organic isocyanate whereby thedistillate contains a major proportion of the organic isocyanate and thestill heel contains the residue component and the tar-thinning agent.

2. A process according to claim 1 wherein said solvent is removed fromthe resulting composition by distillation prior to Step (B).

3. A process according to claim 1 wherein the organic isocyanate istoluene diisocyanate and said solvent is orthodichlorobenzene.

4. A process according to claim 1 wherein the tarthinning agent is anaromatic petroleum oil having less than 35 Weight percent saturates.

5. A process according to claim 1 wherein the weight ratio oftar-thinning agent to residue component is about 0.7/1 to 1.5/1.

6. A process according to claim 1 wherein Step (C) is carried out at atemperature of about l00l C. and

a pressure of about 05-50 mm. l-lg.

7. A process according to claim 6 wherein a stripping agent is used inremoving the phosgene and hydrochloric acid, the stripping agent used inremoving the organic isocyanate in Step (C) is orthodichlorobenzene, andStep (C) is carried out at a temperature of about l30-165C. and apressure of about 2-20 mm. Hg.

8. A process according to claim 1 wherein Step (C) is carried out bypassing a continuous stream of the mixture obtained in Step (B) and thestripping agent into distillation means capable of continuously removinga major proportion of the organic isocyanate, and passing a continuousstream of distilled isocyanate and stripping agent out of thedistillation means.

2. A process according to claim 1 wherein said solvent is removed fromthe resulting composition by distillation prior to Step (B).
 3. Aprocess according to claim 1 wherein the organic isocyanate is toluenediisocyanate and said solvent is orthodichlorobenzene.
 4. A processaccording to claim 1 wherein the tar-thinning agent is an aromaticpetroleum oil having less than 35 weight percent saturates.
 5. A processaccording to claim 1 wherein the weight ratio of tar-thinning aGent toresidue component is about 0.7/1 to 1.5/1.
 6. A process according toclaim 1 wherein Step (C) is carried out at a temperature of about100*-180*C. and a pressure of about 0.5-50 mm. Hg.
 7. A processaccording to claim 6 wherein a stripping agent is used in removing thephosgene and hydrochloric acid, the stripping agent used in removing theorganic isocyanate in Step (C) is orthodichlorobenzene, and Step (C) iscarried out at a temperature of about 130*-165*C. and a pressure ofabout 2-20 mm. Hg.
 8. A process according to claim 1 wherein Step (C) iscarried out by passing a continuous stream of the mixture obtained inStep (B) and the stripping agent into distillation means capable ofcontinuously removing a major proportion of the organic isocyanate, andpassing a continuous stream of distilled isocyanate and stripping agentout of the distillation means.